The present invention relates to hydraulic pumps which serves as a power source for use, e.g. in power steering devices for motor vehicles.
One of the hydraulic pumps of this type is disclosed, e.g. in JP-A 7-279871. This hydraulic pump comprises a pump unit arranged between a pump body and a pump cover, and a drive shaft supported by a bearing bush arranged through a bearing hole of the pump body to drive the pump unit. A seal chamber is arranged at an end of the bearing hole.
The bearing bush, which is obtained by rounding a plate, has at the inner periphery a spiral oil groove which open to both ends, through which hydraulic oil leaking from the pump unit is led into the seal chamber.
With this known hydraulic pump, the pump unit is driven by the drive shaft supported by the bearing bush, i.e. through a pulley mounted to an end of the drive shaft which protrudes from the pump body, and a belt wound on the pulley, obtaining the pump function.
With driving of the pump unit, leakage oil is produced from the pump unit, which is led from the bearing hole into the oil groove. Hydraulic oil in the oil groove of the bearing bush fulfills lubrication between the bearing bush and the drive shaft, then flows into the seal chamber. Lubrication between the bearing bush and the drive shaft is ensured by a predetermined clearance arranged between the bearing bush and the drive shaft. Lubricating oil is supplied from the oil groove to the clearance, which forms an oil film with rotation of the drive shaft to support the drive shaft, avoiding direct contact of the drive shaft with the bearing bush.
When the drive shaft drives the pump unit, the drive shaft is supported by the bearing bush through the predetermined clearance arranged between the two, and thus it can be inclined in the cylindrical bearing bush. As a consequence, the bearing bush comes in contact with the drive shaft with greater pressure at both ends, but with smaller pressure substantially in the center portion as viewed in the longitudinal direction. It is thus understood that with lubrication between the bearing bush and the drive shaft, stable formation of an oil film is necessary at both ends of the bearing bush than substantially in the center portion thereof.
With the known hydraulic pump, however, if the bearing bush obtained by rounding a plate has a seam corresponding to a portion with which the drive shaft comes in contact with greater pressure, the seam interrupts formation of an oil film, which may cause poor lubrication, particularly, at both ends of the bearing bush. Moreover, due to the fact that the bearing bush has at the inner periphery an oil groove formed overall as viewed in the longitudinal direction, through which hydraulic oil lubricates the inner surface of the bearing bush then flows into the seal chamber, the oil groove also interrupts formation of an oil film, which may cause poor lubrication at both ends of the bearing bush.
Specifically, when the drive shaft rotates, lubricating oil is drawn in a wedgelike way into the clearance formed between the drive shaft and the bearing bush to be narrower in the direction of rotation, which produces an oil-film pressure to achieve formation of an oil film. However, existence of a seam and an oil groove of the bearing bush prevents achievement of a sufficient oil wedge action, making stable formation of an oil film difficult, resulting in possible poor lubrication.
It is, therefore, an object of the present invention to provide hydraulic pumps which are free from poor lubrication between the drive shaft and the bearing bush.